A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined.
Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches, and principal component analysis.
The targets used by some scatterometers are relatively large gratings, e.g., 40 μm by 40 μm, and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). This simplifies mathematical reconstruction of the target as it can be regarded as infinite. However, in order to reduce the size of the targets, e.g., to 10 μm by 10 μm or less, so they can be positioned in amongst product features, rather than in the scribe lane, metrology has been proposed in which the grating is made smaller than the measurement spot (i.e., the grating is overfilled). Typically such targets are measured using dark field scatterometry in which the zeroth order of diffraction (corresponding to a specular reflection) is blocked, and only higher orders processed. Diffraction-based overlay using dark-field detection of the diffraction orders enables overlay measurements on smaller targets. These targets can be smaller than the illumination spot and may be surrounded by product structures on a substrate. Multiple targets can be measured in one image.
In the known metrology technique, overlay measurement results are obtained by measuring the target twice under certain conditions, while either rotating the target or changing the illumination mode or imaging mode to obtain separately the −1st and the +1st diffraction order intensities. Comparing these intensities for a given grating provides a measurement of intensity asymmetry in the grating, and intensity asymmetry in an overlay grating can be used as an indicator of overlay error.
Although the known dark-field image-based overlay measurements are fast and computationally very simple (once calibrated), they rely on an assumption that overlay is the only cause of intensity asymmetry in the target structure. Any other asymmetry in the stack, such as asymmetry of features within one or both of the overlaid gratings, also causes an intensity asymmetry in the 1st orders. This feature asymmetry, which is not related to the overlay, clearly perturbs the overlay measurement, giving an inaccurate overlay result. Feature asymmetry in the bottom grating of the overlay grating is a common form of feature asymmetry. It may originate, for example, in the geometrical shape of the grating, or in the substrate processing steps such as chemical-mechanical polishing (CMP), performed after the bottom grating was originally formed. The feature asymmetry may thus include process asymmetry, where imperfections are introduced in the steps of processing the grating, or more generally, the substrate.
Methods have been found for taking the feature asymmetry into account when determining the overlay from the intensity asymmetry. However, this method still includes inaccuracies when other causes of intensity asymmetry are present such as feature asymmetry. Thus, there is room for a method to reduce inaccuracy in an overlay estimation based on this method.